1. Field of the Invention
The invention relates to shielded enclosures and, more particularly, is directed to enclosures, cable entry ports and assemblies and methods for protecting electrical and electronic equipment from damage caused by electromagnetic fields created by lightning strikes and other electrical or electronic equipment or sabotage.
2. Description of the Invention Background
Perhaps when one thinks of the discovery of electricity, various depictions of Benjamin Franklin flying his infamous kite in a lightning storm come to mind. While history is somewhat ambiguous as to exactly what role Franklin's observation of lightning played in the discovery of electricity and the subsequent invention of the myriad of electronic devices that we use in our every day life, man still struggles to harness and control its power.
A lightning strike starts with a “local” electrical breakdown of the atmosphere. Typically, the lightning “steps down” toward the earth about 150 feet in one micro-second time increments every 49 micro-seconds. During each of the 49 micro-second dormant stages, an imaginary hemisphere having a radius of 150 feet can be used to determine the next jumping distance. Any object such as a tower, building, tree, etc. which penetrates this hemisphere, can be chosen as the point of attachment for the return stroke (i.e., the lightning strike).
When lightning strikes an object, it generally takes the path of least resistance to the ground and, if that path happens to pass through a circuit that includes electrical/electronic components incapable of accommodating such a surge of electrical current, those components may be damaged or destroyed. For example, many people have had electrical devices such as televisions, VCRs, computers, etc. damaged by a lightning strike or probably know someone that has experienced such damage.
Over the years, a variety of different devices and methods have been developed to prevent electrical component damage caused by a lightning strike. Examples of such devices are fuses, circuit breakers, surge protectors, etc. Another device commonly used to protect electronic equipment from lightning strike damage is known as a lightning arrester which reduces excessive voltage resulting from lightning to a safe level by “grounding” the discharge.
“Ground” is the term used to describe the common connection in an electrical or electronic circuit. The common connection for electronic circuits is almost always ultimately routed to the earth. The earth is a fair to good conductor of electricity depending upon the characteristics of the soil. A ground connection is the electrical contact between the common point of an electrical or electronic system and the earth. Effective grounding systems typically include one or more ground rods driven into the soil to a depth of 6 to 8 feet. Other common grounding methods involve establishing a ground connection with a structure's cold water pipe. Thus, some circuit protection apparatuses and methods serve to provide a deliberate and controlled path for the electrical energy resulting from a lightning strike to return to ground.
While the above-mentioned apparatuses and practices serve to protect electrical components from surges of electrical energy caused by lightning strikes, lightning can cause other problems which require different solutions. For example, as the electromagnetic field, created by a “nearby” lightning strike passes through electrical/electronic components, voltages and/or currents are generated that may cause permanent damage to the components. In the past, to protect electrical/electronic components from such damage, each individual component was housed within its own enclosure fabricated from a material that is capable of attenuating electromagnetic effects by providing a low-reluctance path for magnetic lines of force and a closed conductive shell for electric lines of force. Because such material can be relatively expensive, the ability to protect numerous components with their own individual enclosures can be cost prohibitive. Also, in some cases, a single shield will reach saturation magnetization because of high-flux density magnetic fields, which reduces the effectiveness of the shield. That problem has been addressed by increasing the thickness of the shield material or by nesting a number of thinner shields together. Either method, however, can be very expensive when numerous electrical/electronic components must be shielded.
Perhaps the industry that has been most plagued with problems associated with lightning strikes is the natural gas industry. Today, the natural gas industry typically utilizes very sophisticated electronic equipment for monitoring and recording the output of gas wells and the purchase and resale of natural gas in the transmission and distribution systems. Usually such equipment is housed within an enclosure or building at the well site or pipeline. In remote locations, such equipment may communicate, via satellite, with computers and other equipment located at the gas company's offices.
In the past, gas companies have enclosed their gas well and pipeline equipment in makeshift buildings fabricated from various materials such as, for example, fiberglass, plastic, cement blocks, corrugated steel, etc. All of those materials generally have poor grounding capabilities in addition to various other shortcomings. For example, while buildings and enclosures fabricated from plastic and/or fiberglass do not experience corrosion problems and are generally easy to erect and transport, they are susceptible to pest and vermin damage. Such enclosures also fail to shield equipment from electromagnetic fields. Likewise, while enclosures fabricated from concrete blocks are generally impervious to vermin and pests, they are difficult to transport and usually must be constructed on site. Such buildings also offer little protection from electromagnetic fields. Enclosures and buildings fabricated from corrugated steel are generally easy to erect, but are susceptible to corrosion and vermin damage while providing little protection from electromagnetic fields.
The problems associated with lightning strikes and the electrical/electronic component damage caused by the electrical and magnetic energy created thereby are not unique to the natural gas industry, however. These problems are encountered in a variety of other industries and applications where electrical/electronic components are susceptible to lightning damage. U.S. Pat. No. 5,749,178 discloses a shielded enclosure that solves many of such problems.
Also, electronic equipment that is vital to an area's telecommunication and utility services may be susceptible to radio frequency and electromagnetic interference caused by adjacent equipment or even caused by sabotage if it is unshielded.
In addition, microprocessor based equipment such as variable frequency drives and computer controlled equipment used within manufacturing plants may be susceptible to such electromagnetic interference.
All of the problems discussed above resulting from electrical or electromagnetic interference may, at one time or another, potentially detrimentally affect electronic equipment regardless of whether the equipment is housed in a building or other structure. If the structure has power cables entering it, those power cables provide convenient avenues for electronic or electromagnetic interference caused by lightening strikes or other external sources to enter the building or structure to detrimentally affect the electronic equipment therein. While some power cable entry port arrangements have been developed in an effort to transport unwanted electricity flowing on entry cables to ground, such prior ports still permit large amounts of such unwanted electricity to enter the structure on the cables. For example, some prior attempts to prevent electrical current resulting from a lightning strike from entering an enclosure on the power cable involved clamping or otherwise attaching a groundwire to the conductor within the cable. However, such approaches still permit significant amounts of current to enter the building or enclosure on the cable.
Thus, there is a need for shielded enclosures that are economical to manufacture and use to prevent equipment from damage caused by electromagnetic fields be it naturally or artificially generated.
There are further needs for cable entry ports and assemblies that are more effective than prior cable entry port arrangements for preventing unwanted electricity caused by lightening strikes and other external sources from entering a building, structure, enclosure, etc. on the cables that enter such buildings, structures and enclosures.